Marcus Weston strides into his office, housed in an elegant Georgian house in the West End of London, clutching a bottle of slime-coloured water. It’s made from lemon, ginger, cucumber and super greens powder, and West drinks 1.5 litres of it a day. “It gives me an extra gear, cleans me inside and makes my thinking clearer,” he says.

Weston, a former investment banker, devotes this clarity to studying and teaching Kabbalah. Commonly described as a mystical offshoot of Judaism, it has grown exponentially in the UK since its London headquarters opened in 2002. Now 1,100 students cram into its premises, which are next door to the upmarket Oriental Club, each week, with waiting lists for some classes and events, and it teaches spirituality to businesses, diplomats, charities and local authorities. Weston has been invited to a Whitehall meeting to discuss the idea of incorporating emotional intelligence content into the national curriculum. “We’re inundated,” he says.

But recently two people came to the London Kabbalah centre who were not seeking to fill a spiritual void in their lives. Knocking on the door at 5am on a Sunday morning were a pair of council officials investigating a call from a local resident, who complained about 36 hours of continuous loud chanting that had emanated from the centre. “I guess it was too noisy for one person, for which I apologise,” says Weston, with a shrug and a smile.

The complaint about the chanting over the Jewish holiday of Sukkot drew fresh attention to a movement whose many reported celebrity adherents include Madonna, Gwyneth Paltrow and Demi Moore. Princess Eugenie, currently eighth in line to the throne, has been photographed apparently wearing the distinctive red thread that devotees ties around their left wrist, although Weston declines to be drawn on this, saying only that he has “many connections with many royal families around the world”.

Weston is a charming man, wearing a smart suit, perfectly judged stubble and an almost-continuous smile. He is happily expansive on the spiritual meaning of Kabbalah, with only a slight clench of the jaw betraying irritation at questions regarding the financing of the movement and claims by some that it is a cult. Brought up in a “very un-observant but culturally Jewish” family, he embarked on a career in investment banking before chancing upon Kabbalah 16 years ago.

“I wasn’t looking for anything spiritual, but felt there was more to life than sitting on the tube. I came to a class and sat at the back, thinking, ‘What on earth am I doing?’. But the more the guy talked, the more it made sense, it resonated.”

Now he is the full-time lead teacher at the London Kabbalah centre, living with his wife and small children in Kabbalah accommodation, eating Kabbalah food and drawing on his savings for all other necessities or pleasures. He says he is entitled to a salary or allowance, but chooses not to take one.

“When I started, there were maybe 10 people in a class. Now we have around 1,100 active students. There are waiting lists, we don’t have enough teachers.” The centre – a branch of the Kabbalah Centre International, founded in New York in 1969 by Rabbi Philip Berg – has secured planning permission to double its size with a £5m extension that will take up to two years to complete – a plan unlikely to endear the Kabbalists to their noise-sensitive neighbours. Weston attributes the growth in interest to “a void that religion can’t fill, but is still needed in society. That void can be filled with something universally spiritual.”

Asked to be more specific in defining Kabbalah, his response is at times hard to follow. “Kabbalah is original spiritual teaching before religion was on the scene. It’s where all monotheistic religions have common ground,” he says. Later he adds: “When we say Kabbalah is a spiritual wisdom, it sits diametrically opposed to religion. In religion, you can blame someone, or God, because ultimately God … is a punishing or rewarding energy or person. In spirituality, it doesn’t exist. In spirituality, it is 100% a spiritual fabric of causes and effects, meaning that each word, thought and action that you plant has an energy to it.”

And again: “Kabbalists say that creation was a point in time when a higher power gave everything of it to the building and creation of the physical world. Since that point, that higher power is out of the game. And therefore the whole revelation of that light or fulfilment, happiness, joy, peace is of our doing. In other words, our actions can reveal or conceal that light. That light sits in our potential.”

Central to Kabbalah is the reading of the 23-volume Zohar, or Book of Splendor, which explains the secrets of the Bible and, indeed, every aspect of life. That it is written in ancient Aramaic is not an insurmountable problem: devotees are instructed to scan the text with a finger in order to absorb the teaching of the book.

Weston now describes himself as a “fully observant Jew”, and indeed wears a yarmulke, a Jewish skullcap. This he describes as a “source of energy”, a conduit for an “infinite light power” that “streams through the cosmos”. But, he swiftly adds, despite Kabbalah’s associations with Judaism, it is “not a Jewish study centre”, but an inclusive, cosmopolitan university.

A request to observe an introductory Kabbalah class is declined by Weston’s PR man, Andrew Michael – whose email announces him as “senior celebrity publicist” – on the grounds that “for some people, spirituality can be quite a private thing”. Instead, Michael offers a list of students willing to be interviewed.

One is Gavin Juniper, a 51-year-old entrepreneur, who first came to the centre in May. Recently separated from his wife, Juniper decided to take his life in hand: give up drinking, lose some weight and find a spiritual dimension that he’d been aware of lacking for some time. In his first class, “there was definitely a connection”; by the time he was on to his sixth class, “I felt like I’d come home.” Kabbalah, he says, makes him feel less stressed and “softens ego. Ego is really the dark side of everything. If you can lose your ego, you become more spiritual.”

Another student, Nimrit Chahal, 40a non-practising Sikh, began attending in June after spending a weekend with other Kabbalists for a wedding. “I wanted to make myself a better person. Kabbalah can give you tools to help with day-to-day life, like how to deal with a situation at work. It’s practical and spiritual.” And, she repeats several times, it is fun.

Not everyone agrees. Claims abound that Kabbalah is a cult that exploits people financially and psychologically, with many of the allegations documented by the Cult Education Institute. These include threats of violence, claims to cure cancer, misuse of funds, legal action against the global Kabbalah movement,fraud and tax evasion. Inform, the London School of Economics-based Information Network on Religious Movements, has produced an analysis of Kabbalah, which records “criticism from a variety of people and organisations who have accused Berg’s centre of ‘indiscriminate outreach, aggressive recruitment methods and exploitative treatment of devotees’”.

It cites exposés of financial aspects of the centre, including allegations of pressure on students to make big donations and of forcing volunteers to work long hours without pay. A BBC documentary in 2005 showed an undercover reporter being charged £860 for “Kabbalah water”, books and a meal, amid suggestions that his cancer could be cured. Weston brushes off such claims. The London Kabbalah centre is a registered religious charity, financed by course fees, its bookshop and donations from students and benefactors, he says. Its West End headquarters are owned by the charity. It offers an introductory 10-class course for free, encouraging participants to contribute to its “scholarship fund”. Other courses and classes are charged at around £10 an hour. “Kabbalah water” costs £2 for a one-litre bottle; a pack of seven red wrist-strings costs £18. The string is to ward off the “evil eye”, says Weston, and the water is “not snake oil, just plain water which hopefully has some energy because it’s around good people”.

He confirms that Kabbalah teachers are unpaid, working full-time in return for accommodation, food and a modest allowance. “I don’t think it’s necessary to state what the allowance is, and it’s different for every person, but it’s more than enough to enjoy London,” he says.

Questions about finances and celebrity supporters are his “main frustration”, he says. “We have 1,000 people a week, we see all sorts of miracles, and all anyone wants to know about is Madonna and Kabbalah water.” Ah, yes. Madonna is a regular visitor to the centre when in London, he says. “She’ll walk into a class or event and sit next to a random person – who might text a friend to say, ‘Guess who’s sitting next to me?’. But everyone here is respectful.” To his knowledge, no one has ever attempted a selfie with her.

Celebrities are attracted to Kabbalah, he says, because of their “false sense of fulfilment which is very dependent on external energy. The definition of a fickle life is when your source of happiness is external. They come to Kabbalah, and find the source of real fulfilment and happiness is internal.”

Public and private institutions are apparently queueing up to have Weston and other Kabbalist teachers share their insights. Weston says he has taught in “most investment banks in town, many of the Fortune 500 companies internationally, in Switzerland for private equity companies” as well as at public-sector bodies. “I’ve done some actually extraordinary things.”

Can he name names? No, but he promises to email with more details when he’s cleared his client confidentiality lines. The information never comes.

• This article was amended on 27 October 2015 to change a reference to “spiritualism” to “spirituality”.